Electronic multiplier for terrain avoidance radar system



J. A. MOULTON Marh 15, 1966 ELECTRONIC MULTIPLIER FOR TERRAIN AVOIDANCERADAR SYSTEM Filed July 1'7, 1962 6 Sheets-Sheet 1 ATTOR N EY March 15,1966 J. A. MOULTON 3,241,068 E ELECTRONIC MULTIPLIER FOR TERRAINAVOIDANCE RADAR SYSTEM Filed July 17, 1962 6 Sheets-Sheet 2 f5 J O 'J fr2 m LLI O g Z 1 I l I o fR fR SYSTEM TR|GGER RADAR RANGE TIMEM|cRo-sEcoNDs) FIG. au

,c l E g Ro Z (l) E R LD E E Ol i 0 fR 1R SYSTEM SYSTEM o TRIGGERTR|GGER RADAR RANGE T|ME (MICRO-sEcoNns) FIG. 3b

INVENTOR JAMES A. MOULTON ATTORNEY J. A. MouLToN 3,241,068

6 Sheets-Sheet 5 March 15, 1966 ELECTRONIC MULTIRLIER RoR TERRAINAvoIDANcE RADAR sYsEEM Filed July 17, 1962 March 15, 1966 A. MouLToNELECTRONIC MULTIPLIER FOR TERRAIN AVOIDANCE RADAR SYSTEM 6 Sheets-Sheet4 Filed July 17, 1962 INVENTOR. JAMES A. MOULTON ATTORNEY March 15, 1966J. A. MOULTON ELECTRONIC MULTIPLIER FOR TERRAIN AVOIDANCE RADAR SYSTEMFiled July 17, 1962 6 Sheets-Sheet 5 msm OQ OQ INVENTOR JAMES A. MOULTONATTORNEY March 15, 1966 J. A. MouLToN ELECTRONIC MULTIPLIER FOR TERRAINAVOIDANCE RADAR SYSTEM Filed July 17, 1962 6 Sheets-Sheet 6 ATTORNEYUnited States Patent O 3,241,068 ELECTRONIC MULTIPLIER FOR TERRAINAVOIDAN CE RADAR SYSTEM James A. Moulton, Santa Ana, Calif., assignor toNorth American Aviation, Inc. Filed July 17, 1962, Ser. No. 211,476 12Claims. (Cl. S25-326) which information is sought concerning theterrain,

automatic computing signal means are utilized to provide la signalindicative of the vertical height of an aircraft above the terrain orother obstacle. Such computation involves the `steps of (l) determiningthe elevation boresight angle of the target or obstacle olf the antennaboresight axis, and (2) determining the elevation angle of the boresightaxis relative to the horizon plane, in order to determine the elev-ationtarget angle of the obstacle. Such computation also involves the stepsof (l) determining the slant radar range of the obstacle relative to theaircraft and (2) multiplying the radar slant range by the target angleto determine the distance by which the aircraft will clear the terrainfeature or obstacle. A ramp function signal (that is, one whichincreases in value with time) is triggered or synchronized with themonopulse system trigger. As the ramp function signal increases, itindicates increasing time delay in the radar return signal. Explainedanother way, the ramp signal increases as the radar signal returns fromobjects farther and farther away. The radar return signal, in theelevation channel is a video signal having a frequency spectrum ofapproximately 200 cycles per second to 2 megacycles per second. If asingle pulse is transmitted (as in pulsed radar) a return `signal isreceived from each reflecting point within the area illuminated by thetransmitted energy. The return signal is received at intervalscorresponding to the radar range of the target or terrain obstacle.Additionally, the computed vertical altitude above the terr-ain obstacleis compared with a desired clearance altitude to develop a warningsignal.

Multiplication of two signals has been performed in the prior art bymeans of vacuum tube circuits employing two dual control pentodes havingthe plates thereof commonly connected, each of the signals being appliedto mutually exclusive grids in push-pull circuit. Application of `onesignal causes a differential change in the gain of the other signal,thus producing a plate circuit output signal which is indicative of theproduct of the two signals such as the product of range 'and targetangle. However, such an arrangement does not provide a suitable null orzero output signal during the intervals when either or both of thesignals, such as target angle and range signals are zero. Also, becausedual control pentodes are not precision devices, it is extremelydificult to balance the outputs of the pentodes as the pentodes age, orin the presence of fluctuating `supply voltages. Further, in airborneapplications wherein the device is subjected to the normal vibrationenvironment of the aircraft, inherent microphonic characteristics of thepentodes produce spurious signals of substantial magnitude. Finally,numerous bias adjustments are necessary in order to find initialoperating points that produce satisfactory null responses to null inputsand satisfactory linearity for a finite range of inputs. A terrainavoidance computer employing such multiplication means together With arange signal sawtooth generator, is described 3,241,068 Patented Mar.l5, 1966 rice in co-pending U.S. application, Serial No. l830,675 forTerrain Clearance Radar, filed by William E. Stoney on July 30, 1959,now Patent No. 3,165,740 and assigned to North American Aviation, Inc.,assignee of the subject invention.

The present invention is an improvement of the video multiplierdescribed in the above-described U.S. application, Serial No. 830,675.The present invention has as one of its principal objects themultiplication of a video input signal by another signal by controllingthe gain of the video input signal, rather than generating a thirdanalog signal representing the product of the two signals. Anotherobject of the present invention is to avoid drift and balance problemsby switching in or out circuit elements of a fixed value so as tocontrol gain. Such method may be seen to be a digital or discrete methodof multiplication. Heretofore as explained previously, multiplicationwas in such instance analog in form wherein two signals were multipliedby using multiplied tubes. 1

In carrying out the invention in accordance with a preferred embodimentthereof, there is provided a monopulse receiver having a periodic systemtrigger, and providing the video return of periodic target anglesignals. The time-phase or time delay of the video target angle signalsrelative to the system trigger are' indicative of the target range,While the amplitudes of the video signals are indicative of targetangle. There is provided, in accordance with the invention, circuitmeans for controlling scaling the gain of the video signals discretely,or in steps, or digitally. Switches in cooperation with control meansoperatively connect and disconnect selective circuit means for causingthe scaling of the video gain to be varied in discrete stepsapproximating a ramp function, in response to the system trigger. Thereis further provided compensation switching for reducing variations inoutput impedance during the period of the periodic system trigger.

By means of the Iabove described arrangement, upon transmission of asingle radar pulse, the amplitude of the video signal will be weightedin gain each time a system pulse is sent out and video signals from theilluminated points commence to return. Because the gain weighting of thevideo signal is representative of the range of the various targetsrepresented by such video signal, while the amplitude of the videosignal is representative of the target angle, the resulting attenuatedvideo signal will be representative of the product of the range of eachtarget by its angle. If the elevation channel video information is used,the product of range and elevation angle is obtained. Hence, it is to beappreciated that the device of the invention provides improved signalmultiplication means involving circuit elements having a fixed valuewhich will not destroy nor interfere with the returned video signal.Hence, the upper frequency spectral content of the video signal is notlost, and the performance of the device as a high speed multiplier isnot limited.

Accordingly, it is an object of the subject invention to provideimproved means for multiplying a monopulse system video signal as afunction of range.

It is another object of the subject invention to preserve theinformation in a video signal in multiplying a video signal by radarrange.

It is a further object of the subject invention to provide videomultiplying means that is not susceptible to microphonics.

It is yet a further object of the subject invention to provide discretemeans for controlling the gain of a video signal as a function of time,each time the receiver commences receiving new video return signals.

It is still another object of this invention to provide video multipliermeans that require fewer circuit adjustments and which is easy tomaintain.

These and other objects of the invention will become apparent from thefollowing description taken in connection with the accompanying drawingsin which:

FIG. 1 is an illustration of the geometry of the terrain avoidanceproblem, the solution of which employs the concept of the invention.

FIG. 2 is a functional block diagram of a terrain avoidance monopulsesystem employing the concept of the invention.

FIGS. 3(a) and 3(b) are representative time histories of the severalresponses of the system of FIG. 2.

FIG. 4 is a functional block diagram of a preferred embodiment of theinvention.

FIG. 5 is a series -of representative time histories of the severalresponses of the device of FIG. 4.

FIG. 6 is a schematic diagram of an exemplary monostable controlmultivibrator.

FIG. 7 is a schematic diagram of the free running clock multivibrator, 2digit bistable multivibrator, l digit switch and 2 digit switch of FIG.4.

In the drawings, like reference characters refer to like parts.

In the application of an airborne monopulse radar to terrain avoidancefunctions, it is desired to provide means for controlling an aircraft toa specified clearance height he above protruding terrain. In order toperform such function most effectively, arrangement must be made todetect such terrain obstable and predict the aircraft vertical clearanceheight relative thereto. This of course must be done beforehand and froma safe range or distance within which necessary maneuvering of theaircraft may be effected. Accordingly, in a terrain avoidanceapplication, the height h is measured from a reference plane 12 down toall iluminated objects which will reflect radar energy and are aboveclearance plane 14. Only those obstacles protruding above the selectedclearance plane, will be indicated by the radar.

Referring to FIG. l there is illustrated the geometry of problem, thesolution of which employs the con-cept of the invention. A low ilyingvehicle 10 carrying a monopulse radar including an elevation angledetection system, with the boresight of the antenna directed as depictedby the line 11. Line 12 indicates the reference plane which may or maynot be the flight path of the vehicle. In general, it does so in orderthat the clearance of the aircraft (if its course is continued) isindicated. Reference numeral 13 indicates a terrain obstacle such as ahill which protrudes above a preset clearance plane 14 which is selectedat some preset distance ho below the reference plane. The depressionangle of the antenna is indicated by the symbol N, and comprises thevertical angle between the reference plane 12 and the antenna boresight11. In a terrainavoidance system of the type described in theabove-mentioned U.S. patent application, Serial No. 830,675, theelevation of the antenna boresight is adjusted so as to intersect withthe clearance plane at a xed slant range distance Ro illustrated in FIG.1 as occurring at point 15. The angle between the antenna boresight, orcenterline and a radar-reflecting target at range R, together with thecomputed vertical distance h between the target and the reference planeare employed in the terrain avoidance computation.

The perpendicular distance h from the reference plane to the ground isexpressed as follows:

h=R sin (N-l-) (1) However, for the combination of ranges and clearancedistances involved in the use of terrain clearance systems, the sinefunction of an angle is approximately equal to the angle itself inradians. Hence,

1=R (N-l-) (2) The warning signal, W, in a terrain avoidance system isdefined as:

Hence, where the obstacle lies below the clearance plane (h l10), nowarning signal occurs.

It is to be observed that the computation of clearance height, h, of thereference plane above the obstacle employs the product of the radarrange of the obstacle and the target angle subtended by the obstacle andthe reference plane, measured from the monopulse antenna (or aircraft).The derivation of the video signal indicative of the target angle (N-l-)is well understood in the art and is described, for example, in theabove-mentioned U.S. patent application, Serial No. 830,675. Suchinformation is derived, for example, from the sum and difference signalsEs and Ed respectively (in a sum. and diierence type angle detectionmonopulse system) and by employing the geometrical constants of theterrain avoidance problem. In the angle (N-l-), the component angle, N,is dened by the geometrical constants of FIG. l as:

The angle is derived from the monopulse operation as:

Ed Es 6) Therefore, Equation 3 for the warning signal W, may berewritten as:

Further, the explicit division indicated by Equation 7 may be avoided ina practical mechanization by rearranging Equation 7 with respect to thesum signal ES:

Hence, the terrain avoidance merchanization need only perform thefunctions of addition (or comparison) and multiplication. Observing thatthe terms 1z0/R0 and K1 are fixed factors, such terms are provided forby suitably adjusting the scaling or gain levels of the video monopulsesignals Es and Ed associated with these terms. Accordingly, the scaledEs and Ed video signals may be summed by a resistive summing network atthe input to a video amplifier, and the multiplication of the summedvideo signals ha RoEs'l'KiEd by radar range R accomplished by means ofthe system shown in FIG. 2.

Referring to FIG. 2, there is illustrated a terrain the sum signaloutput terminal 21 of monopulse system 17, and a second terminal ofsecond summing resistor 19 is connected to the elevation differencesignal output terminal 22 of monopulse system 17.

The construction of video amplifier 20 and monopulse angle detectionsystem 17 is well known in the art, and is described, for example, inthe above-mentioned U.S. Patent application, Serial No. 830,675 and inU.S. Patent No. 2,933,380 for an Integrated Aircraft and Fire ControlAutopilot, issued April 26, 1960, to John R. Moore et al.

The summed video output from amplifier 20 is fed as a video input tovideo multiplier 23 which is comprised of discrete scaling orattenuating means 24, selective switching means 2S, and control means26.

Scaling or attenuating means 24 is connected in circuit with the outputof video amplier 20 for gain weighting the video signal in digital ordiscrete steps in response to selective switching means 25 withoutchanging the identity of the video signal input. In other words, thegain level of the video signal is changed in digital or discrete steps,rather than generating an additional or analog signal which is afunction of the video input signal to scaling multiplier 23. Selectiveswitching means 25 is responsively connected to control means 26 forselectively switching the discrete scaling means in such a fashion as toprovide a digital or discrete gain function which approximates a rampfunction within the period of the periodic system trigger of monopulsesystem 17. Control means 26 is responsively connected to the systemtrigger of monopulse system 17 for synchronizing the operation ofmultiplier 23 therewith, for reasons which will be more fully explainedin connection with FIGS. 3(a) and 3(b).

Referring to FIGS. 3(a) and 3(11) there are illustrated representativesignal responses of the device of FIG. 2. In normal operation of thedevice of FIG. 2, the video signal input to multiplier 23 (e.g., targetangle signal from video amplifier 20) has a magnitude Vt indicative ofthe target angle (N-l-) of Equation 2, and occurs at a time phase, fR,relative to the system trigger which is indicative of the range, R, ofsuch target or terrain obstacle, as shown in FIG. 3(11).

Scaling means 24, selective switching means 25, and control means 26 ofFIG. 2 cooperate to provide a discrete signal level or discrete gainweighting which resembles a staircase and approaches a ramp function upto a maximum gain level or gain weighting at time tRO, corresponding tothe maximum range R0 of Equation 8, within the period of the systemtrigger; and then the gain collapses to zero volts output per voltinput, as shown in FIG. 3(b). Hence, it is to be appreciated that thegain of scaling means 24 in FIG, 2 varies as a function of time as tosimulate radar range. Further, the time phase occurrence of the targetangle video signal Vt which is applied to scaling means 24, correspondsto the radar range of the target, which causes such signal. Accordingly,the video input signal to multiplier 23 is gain weighted by a factorindicative of the target range whereby the attenuated signal isindicative of the product of target angle and target range described inEquation 8. In other words, the gain of the video signal is adjusted asa discrete function of the time phase of such video signal.

The means of obtaining such described mode of operation of themultiplier 23 of FIG. 2 is to be more fully appreciated from aconsideration of the block diagram of the device shown in FIG. 4.

Referring to FIG. 4, there is illustrated a block diagram of a preferredembodiment of the multiplier of FIG. 2. There is provided a source 20aof a periodic video signal, the amplitude and time phase of which areindicative of target angle and target range respectively. There isfurther provided means for adjusting the gain of the video signal fromsource 20a as a function of the time phase of the video signal. Suchgain adjusting means is comprised of a plurality of signal attenuatingand switching networks having inputs commonly connected in circuit toamplifier 20a, for providing binary-coded relative scaling of the videooutput therefrom; and further providing switching of such mutuallyrelatively scaled video signals. Six such discrete scaling means 29, 30,31, 32, 33 and 34 are shown, being designated as a l-digit switch,2-digit switch, 4-digit switch, 8-digit switch, 16- digit switch, and32-digit switch, respectively.

Each digit switch is similarly constructed and com- -prises a seriesinput scaling resistor 35 and parallel output scaling resistor 36 forproviding scaling or gainweighting (e.g., attenuation) of the videosignal input, the series resistor 3S being connected in series betweenthe video input land output terminals 37 and 38 of the digit switch, andthe parallel resistor being connected across the o-utput of the digitswitch. The output of each digit switch is summed at the input of aninverter amplitier by means of a mutually exclusive summing resistorinterconnecting the output of an associated digit switch and the inputof inverter amplifier 40. For example, the signal output of the l, 2-,4-, 8-, 16- and 32-digit switches is summed by means of video summingresistors 41, 42, 43, 44, 45 and 46 respectively. The resistance valueselected for the summing resistors is chosen to be high enough relativet=o the attenuation network as to provide relative impedance isolation.The values selected for series and parallel resistors 35 and 36 in eachdigit switch are chosen to provide a relative gain weighting betweendigit switches in the successive binary-code radio Switching of thebinary code scaled signal output of each digit is provided by means of aswitching transistor 47 having its emitter-collector circuit connectedacross parallel resistor 36, and having its base or control electroderesponsively connected to control means for selectively short-circuitingthe binary-scaled video signal output from the digit switch.

The control means of the device of FIG. 4 is comprised of a like numberof se-rially-connected bistable multivibrators as digit switches lessone, a monostable control multivibrator 57 responsively connected to thesystem trigger signal source of FIG. 2, and .a clock control means 58responsively connected to control ymultivibrator 57 and the last one ofthe serially connected bistable multivibrators for providing a clockcontrol signal.

There is further provided a free-running clock multivibrator 59responsive to the control signal from clock control means 58, andoperatively connected to drive the first one (e.g., element 52) ofserially-connected bi-stable multivibrators 52, 53, 54, 55 and 56.

Monostable control multivibrator 57 may be comprised of a so-calledone-shot multivibrator or other means similarly well known in the artfor providing a two-state, monostable circuit element, and accordinglyis shown in block form only. The construction of free runningmultivibrator 59 and bistable multivibrators 52, 53, 54, 55 and 56 issimilarly well-known in the art, and these elements are therefore alsoshown in block form only.

The one-set youtput from each of elements 59, 52, 53, 54 and 55 isconnected to trigger or drive the succeeding one of elements 52, S3, S4,55 and 56; and is further connected to the 'base of the switchingtransistor 47 of an associated digit switch. For example, the one-setoutput of clock multivibrator S9 is operatively connected to the base oftransistor 47 in l-digit switch 29, and to the input of 2digit bistablemultivibrator 52; the output of 2.- digit multivibrator 52 is similarlyoperatively connected to 2-digit switch 30 and the input of 4-digitmultivibrator 53. The output of 4-digit multivibrator 53 is operativelyconnected to .the associated 4-digit switch 31 and S-digit multivibrator54, and so forth. It is to be noted that the output of the last one ofthe serially-connected bi-sta'ble mulivibrators (e.g., 32-digitlmultivibrator 56) is operatively connected to the associated 32-digitswitch 34; and

is furthe-r connected to clock-control means 58 for shutting otffree-running multivibrator 59 when the last bistable multivibrator,element 56 is triggered, for reasons which will be more fully explainedhereinafter.

Clock control means 58 may be comprised, for example, of a controltransistor having its base or control electrode responsively connectedto the output of multivibrator 57, and including means forforward-biasing of such transistor by the feedback signal from triggeredmultivib-rator 56.

Operation of the device of FIG. 4 may be more easily -understood byreference to FIG. 5.

Referring to FIG. 5, there are illustrated representative time historiesof the responses of several elements of the device of FIG. 4 over theperiod yof `a single cycle of the periodic system trigger. Curve 61represents the system trigger input to control multivibrator 57. Curve62 represents the output response of monostable multivibrator 57 to thesystem trigger, illustrating a bi-polar time constant or time delay ofmore than onehalf cycle in returning to the stable state (eg, state attime before to), upon removal of the system trigger excitation or input.The purpose of the time constant is to cause control multivibrator 57 tokeep clock control means 58 of FIG. 4 turned on until a feedback signalfrom multivibrator 56 (e.g., response curve 63) can be employed duringthe rest of the cycle for control signal purposes, as will be more fullyexplained hereinafter.

Curve 65 represents the response of free-running multivibrator 59 tocontrol means 58, showing the excitation of such response or output uponthe occurrence of the system trigger signal (curve 61). Curve 66represents the l-set response of bi-stable multivibrator 52 to theoutput of clock multivibrator 59, demonstrating the frequency divisionor change of state (c g., alternation between the two olf and on states)occurring in response to the change from on to off state of clockmultivibrator 59 (curve 65). Similarly, curves 67, 68, 69 and 70represent the time histories of the l-set outputs of 4-digit, 8-digit,16-digit and 32-digit Ibi-sta'ble multivibrators 53, 54, 55 and 56respectively, illustrating the successive frequency division or changeof state of each such multivibrator occurring in resp-onse to the changefrom on to 01T state of the preceding multivibrator output or drivingsignal. While curve 70 represents the time history of the l-set outputof 32-digit multivibrator 56, it is to be appreciated that curve 63,being the response of the O-set response of multivibrator 56, is themirror image of curve 70.

Further, it is to be appreciated that the O-set response depicted bycurve 70 provides a second control signal input to control means 58 ofFIG. 4 whereby the clock control output signal (curve 64) remains oneven after the duration of the input signal (curve 62) from controlmultivibrator 57 of FIG. 4. Hence, the response of clock control means58 (curve 64) remains on during the occurrence of the O-set inputs fromeither or both of multivibrators 56 and 57 (curves 63 and 62,respectively, between times to and im), and changes to the off stateupon the concurrent oif state of both of them (e.g., between time tm andthe occurrence of the subsequent system trigger).

Upon control means 58 of FIG. 4 switching to the off state, clockmultivibrator 59 is stopped in the off state, stopping the successivefrequency division by successive ones of serially-connected bi-stablemultivibrators 52, 53, 54, 55 and 56. For the reason that the clock isstopped at the concurrence of the off state of the 1set of 32- digitmultivibrator, and because the 1set output of each successive bi-stablemultivibrator switches to the olf state upon the occurrence of the offstate of the preceding multivibrator, all of the digit controlmultivibrators 52, 53, 54, 55, 56 and 59 of FIG. 4 are stopped with thel-state outputs in the off condition, as indicated by curves 65, 66, 67,68, 69 and 70 at time tw. Hence, it is to be understood that themultivibrators are all reset to the condition depicted in FIG. 5 beforetime to.

Curve 71 of FIG. 5 represents a time history of the relative gain orweighting of the device of FIG. 4, as measured between video input line28 and video output line resulting from the cooperation of the digitswitches with the associated multivibrators and the summing network ofFIG. 4 whereby the cyclic addition of selective combinations of binaryscaled outputs from the digit switches produces a relative gainweighting of the video signal input which varies by discrete stepsapproximating a ramp function. At lo, l-digit switch 29 is turned on bymultivibrator 59, the other digit switches 32, 33, 34, 35 and 36remaining off, providing a relative gain weighting of 1 to the videosignal transmitted through digit switch 29 to the input of amplilier 40.At t1, multivibrator 59 turns off l-digit switch 29, multivibrator 52turns on 2-digit switch 30, providing a relative gain weighting of "2 tothe video signal transmitted through digit switch 30 to amplifier 4i).At t2, multivibrator 59 turns on l-digit switch 29, providing a l gainweight of 1, in addition to the gain weighting of "2 provided by digitswitch 30, these two gained weighted video signals being summed bysumming resistors 41 and 42 to provide a total gain weighting of 3 attime t2.

Similarly, at time t3, multivibrators 59 and 52 turn off switches 29 and30 respectively, and multivibrator 53 turns on 4-digit switch 31 wherebya relative gain weighting of 4 is provided to the video input toamplitier 40. At time t4, l-digit switch 29 is turned on, providing inconjunction with 4-digit switch 31, a gain of 5; at time t5, switch 29is turned olf and Z-digit switch 30 is turned on, providing inconjunction with 4-digt switch 31 a gain of 6. At time t5 both l and 2digit switches 29 and 30 are turned on, providing in conjunction with4-digit switch a combined gain weighting of 7, representing the sum ofthe gain weightings provided by each switch singly. At successiveincrements of time, representing a cycle of the clock multivibratorresponse (curve successive digit switches are similarly lsequentiallyoperated in conjunction with selected combination of prior-digitswitches being interconnected and arranged as described above, wherebythe elective gain between video input line 28 and video output line ofFIG. 4 is varied in discrete steps as a function of time, as to resemblea staircase and approximate a ramp function.

Hence, it is to be appreciated that the amplitude of the video analogoutput signal applied to the input of amplilier 4t) in FIG. 4 isindicative of the product of the amplitude and time phase of the videoinput signal on line 28 of FIG. 4.

It is to be appreciated that the summing resistors 41, 42, 43, 44, 45and 46 do not provide ideal impedance isolation between the gainweighting video signal channels comprising digit switches 29, 30, 31,32, 33 and 34. Accordingly, the actual gain weighting contributed byeach of the digit switches will be effected by the specific otherswitches being operating in combination with such switch at any instant.Therefore, the scaling accuracy of the device would be limited, even if10% tolerance resistors were used in such structure.

In order to avoid such limitation in the scaling accuracy, auxiliary orcompensating impedance Switching means is incorporated in each digitswitch, whereby the same output impedance is presented in both the offand on states lof the switch. In this way the impedance coupling of therest of the digit switches remains constant relative to any one of theplurality of digit switches (but for switching transients), over theperiod of the periodic trigger, and each one of the digit switches maybe separately and precisely calibrated. The structure for effecting suchcompensation is illustrated in FIG. 4.

Referring again to FIG. 4, and more particularly to l-digit switch 29,there is additionally provided a compensatory shunt resistor 75connected across the emitter and collector electrodes of a compensatoryswitching transistor 76. The combination of elements 75 and 76 isconnected in series with compensatory summing resistor 77 across theinput to amplier 40. The base or control electrode of compensatorytransistor 76 is connected to the 0-set output of associatedmultivibrator 59.

In the cooperation of compensatory elements 75, 76 and 77 with the priordescribed gain weighting and digital switching elements associated withswitching transistor 47, compensatory transistor 76 is switched off bythe complementary -state output of multivibrator 59 when switchingtransistor 47 is switched on, by the l-state output of multivibrator 59.In other words, transistors 47 and 76 are concurrently switched tomutually exclusive states. Hence, where digit switch 29 is on(corresponding to switching transistor 47 being of)', then compensatorytransistor 76 is om thereby connecting compensatory summing resistor 77across the input to amplifier 4t). When the digit switch is oit(switching transistor 47 is on, shorting the output of video signalterminal 38), then compensatory transistor 76 is ofli leavingcompensatory resistors 75 and 77 connected in series across the input toamplier 40.

Now, if the Values of video signal summing resistor 41 and compensatorysumming resistor 77 are selected to be approximately mutually equal, andthe values of the Video signal shunt input resistor 36 and shuntcompensatory resistor 75 are approximately mutually equal, then theoutput impedance presented by l-digit switch 29 will be the same in boththe off and on states. More precisely, the output impedance of thecompensating network for each digit switch in the o state is to be thesame as that of the video gain switching channel associated with thatswitch in the on state. Further, the output impedance of thecompensating network for each digit switch in the on state is to be thesame as that of the video gain switching channel associated with thatswitch in the off state.

Referring to the l-digit video signal channel of FIG. 4, for example, inequation form:

and where R1, R2, R3 and R4 correspond to the resistance values ofresistors 35, 36, 41 and 75 respectively.

Such explicit requirement further impliedly requires or implies thattransistors 47 and 76 are similar inperform ance characteristics.

A preferred circuit diagram of the cooperation of certain blocks of FIG.4, illustrating exemplary circuit values therefor, is shown in FIGS. 6and 7.

Referring to FIG. 6, there is illustrated a detailed circuit schematicdiagram of the circuit arrangement of an exemplary monostable controlmultivibrator 57 and indicating the values of the circuit parametersemployed. Multivibrator 57 is a conventional circuit having aconventional output terminal 80 responsively connected to an outputsignal source S1. However, interposed between signal source 81 andoutput terminal 80 is a rst order lag network comprising series resistor82 interconnecting source 81 and treminal 80, and shunt capacitor 83connected across the output of multivibrator 57.

In response to positive going pulses from a system trigger source toone-shot multivibrator 59, a normally on transistor 84 is turned off;and normally off output transistor 85 is turned on, remaining on for aperiod determined by the time constant of the combination of capacitor86 and base supply resistor 87. This time constant is selected to beequal to more than one half the period of the periodic system trigger,as previously described in connection with one-shot multivibratorresponse curve 62 at t9, illustrated in FIG. 5, for the reasonsexplained in connection therewith. The particular time constant circuitillustrated has been selected for employment with a monopulse radarhaving a nominal range maximum corresponding to 488 microseconds.

The purpose of the first order lag network interposed at the output ofmultivibrator 57 is to provide an initial delay in the output signalcorresponding to a time delay inserted at time t0 in curve 62 of FIG. 5,by an amount corresponding to about one half the pulse time width(Mzzl-t0) of the output from clock multivibrator 59 (e.g,. curve 65 ofFIG. 5). In this way, all of the sequential outputs of the seriallyconnected multivibrators of FIG. 4 are all mutually shifted in time bysuch amount. The reason for such delay is to time-bias the range-gain orcause the tirst range time increment (corresponding to the leading edgeof the unit digit increment of curve 71 in FIG. 5) to occur slightlylater than zero time (to), and hence avoid indicating an erroneous niterange at zero range time.

The delayed output from terminal of multivibrator 57 is fed to clockcontrol means 58 having an output terminal 94, input terminal 97, andcommon output input or ground terminal. Clock `control means 58 iscornprised of switching transistor 88 having the emitter and baseelectrodes commonly connected to the common output-input or groundterminal, resistor 90' being interposed between the ground terminal andthe base electrode.

The input to clock control means 58 from multivibrator 59 is fed to thebase or control electrode of switching transistor 88 through seriesconnected clipping diode 89.

The base or control electrode of transistor 88 is also connected bymeans of line 93 to the l-state output terminal of multivibrator 56, asshown in block form in FIG. 4. The base or control electrode is furtherconnected to a source of D.C. potential through forward-biasingresistors 91 and 92, which cooperate with resistor 90 as a bias voltagedivider network to bias transistor 88 into the on state. Hence, nooutput signal potential occurs on output terminal 94 because thecollector electrode of transistor 88, to which it is connected, isshorted to ground.

In normal operation of the single-shot multivibrator 57 and clockcontrol means 58 of FIG. 6, transistors 84 and 88 are normally on Uponreceipt of a system trigger signal by multivibrator 57, normally ontransistor 84 switches to the ott state, and transistor 85 switches tothe on state, whereby the potential at the collector of transistor 85drops, providing a negativegoing pulse through lag-network elements 82and 83 to input terminal 97 of control means 58. In response to an inputsignal of negative sense applied to either or both of input terminal 97and input line 93, the forward bias on the control electrode oftransistor 88 is overcome, and a switching signal of positive senseappears on output terminal 94 for the duration of such input signal.Upon the removal of the input signals, the output signal ceases.

The ouput ysignal from clock control means 58 is fed to clockmultivibrator 59 shown in FIG. 7.

Referring to FIG. 7, there is illustrated free running clockmultivibrator 59, Z-digit bistable multivibrator 52, 1digit switch 29,and 2digit switch 30, all arranged and mutually interconnected as likereferenced elements of FIG. 4.

Free-running multivibrator 59 is comprised of an input transistor and anoutput .transistor 101, the base of input transistor 100 being connectedto input terminal 98 which, in Iturn, is connected to output terminal 94of clock control means 58. Multivibrator 59 is connected in drivingrelation to the input of bistable multivibrator 52 by means of vaninterconnecting terminal 99.

It is to be appreciated that the forward bias applied to the base oftransistor 88 of FIG. 6 causes transistor 88 to conduct, therebyclamping terminal 94 and the base of transistor 100 to ground, wherebyfree running multivibrator 59 is prevented from running.

In normal operation of the free running multivibrator 59 and thebistable multivibrator 52 of FIG. 7, the base of transistor 100 is offby reason of being clamped to ground by normally on transistor 88 ofFIG. 6. Transistors 101 and 103 are normally on, prior to theapplication of the system trigger to the input of one-shot multivibrator57 in FIG. 6. Upon and during the application of a positive potential toterminal 98 of free running multivibrator 59 (corresponding to turningoff transistor 88 of FIG. 6), a series of negative going pulses areapplied sequentially at Iterminal 99, in synchronism and in time phasecoincidence with positive going pulses applied to the base of videotransistor 47 of l-digit switch 29 (response curve 65 of FIG. 5), whilea series of positive going pulses are applied subsequently to the baseof compensatory transistor 76, at 180 time phase relation tothe pulsesapplied t` the base of transistor 47. The first and second output ofelement 59, applied to the base of transistor 47 and 76 respectively ofelement 29, may be referred to as a l-state output and 0-state outputrespectively, for providing signals indicative of the two mutuallyexclusive states of free-running multivibrator 59, as is will understoodin the art.

Normally off transistor 102 of bistable multivibrator 52 is turned on bythe first one and by alternate successive ones of the negative goingpulses applied to terminal 99 of FIG. 7 (transistor 103, already beingon or saturated, does not respond to such input, but is ultimatelyturned ott by transistor 102). The second negative going pulse andalternate successive pulses subsequent to such negative pulse applied toterminal 99, cause transistor 103 (being then in the off state) to turnon, which action subsequently Iturns olf transistor 102, Ias is wellyunderstood in the bistable multivibrator art. In this way, a negativegoing pulse output having twice the repetition rate of the negativegoing pulse input to bistable multivibrator 52 on terminal 99 isprovided on terminal 104 to multivibrator 53 of the successive stages ofmultivibrators illustrated in FIG. 4.

Further, a series of positive going pulses appear at the base of videoswitching transistor 47 of Z-digit switch 30 (response curve 66 of FIG.5) in phase with the negative going pulse output at terminal 104, and aseries of positive going pulses appear at the base of compensatingtransistor 76 of switch 30 at 180 time yphase relation to those appliedto the base of transistor 47 of switch 30. The rst and second output ofelement 52 -applied to the base of transistor 47 and 76 respectively inelement 30, may be referred to as a l-stalte output and O-state outputrespectively for providing signals indicative of the two mutuallyexclusive states of bi-stable multivibrator 52, as is well understood inthe art.

It is to be appreciated that each of rthe bistable multivibrators 52,53, 54, 55 and 56 of FIG. 4 may be constructed similarly, as toresemble, for example, the exemplary circuit of multivibrator 52 shownin FIG. 7. It is to be further appreciated that the feedback line 93 (inFIG. 4) connected to clock control rneans 58 would be connected to acorresponding output terminal 104 of 32- digit multiplier 56.

It will be seen that the device of this invention provides eicient meansfor discretely varying the gain of la video signal as a 'function of thetime phase of such video signal relative to a syst-em trigger. Hence,novel and useful structure is described for use in a Iterrain avoidancemonopulse radar receiver for generating a terrain clearance signalindicative of the product of the target angle (provided by the magnitudeof a video signal) and range 12 (provided by the time delay of suchvideo signal) of a terrain obstacle.

While the device has been described and illustrated as useful in aterrain avoidance computer, the principle of the invention extends tothe analog multiplication of any two variables by discretely adjustingthe gain of tan analog voltage (representing an analog of one vari-able)as a discrete -function of time (representing 'a second variable).

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample only, and is not to be taken by way of limitation, the spiri-tand scope of this invention being limited only by the terms of theappended claims.

I claim:

1. In a monopulse receiver system which generates a periodic videosign-al having a time-phase relative =to a periodic system triggersignal, means for adjusting the gain of said video signal as a functionof said time phase, comprising: an input terminal subjected to saidIvideo signal from the source thereof, an output terminal, a pluralityof attenuating networks commonly interconnecting said input terminal andsaid output terminal, the output gain weighting of each successiveattenuating network being larger than preceding ones of said attenuatingnetworks, a summing resistor interposed between the output of each ofsaid networks and said output terminal, a switch interposed in circuitcontrolling the actuation of each said attenuating network, clock meansresponsive to said periodic trigger signal for selectively turning onand oit said switches in a predetermined manner during a periodic timeinterval, whereby the video gain at said output terminal is varied in`discrete increments resembling a periodic sta-ircase function.

2. An electronic signaling device responsive to a video signal and atrigger signal Iand having an output terminal comprising: a plurality ofsignal attenuating networks commonly adapted to be responsivelyconnected to a common source of said video signal, the gain ofA suchsuccessive network being substantially equal to the sum of the precedingones of said networks plus the first network, a like number of summingresistors at attenuating networks, each summing resistor interconnectingan output of a mutually exclusive one of said networks and said outputterminal, a like number of switches as attenuating networks, each switchinterposed in circuit across the output of a mutually exclusive one ofsaid attenuating networks, clock means response to said periodic triggersignal for alternately turning on and off said switches during aperiodic time interval, said clock means further including means forselectively controlling successive ones of said switches upon theconcurrence of the off-states of preceding ones of said switches duringsaid periodic time interval, whereby the gain of said signaling deviceis varied in discrete increments resembling a periodic staircasefunction.

3. An electronic signaling device responsive to a video input signal,and a periodic trigger signal and having an output terminal comprising:a video amplifier having an output and adapted to be responsive to saidvideo input signal, a plurality of video summing resistors commonlyconnected to said output terminal, a like number of digit switches asvideo summing resistors, each digit switch interposed in circuit betweensaid video amplifier and a mutually exclusive one of said video summingresistors; said digit switch comprising a gain-attenuating networkresponsively connected to the output of said video amplifier, andswitching means connected across `the output of said gain attenuatingnetwork for short circuiting said gain attenuating network output; andcontrol means responsive to said periodic trigger signal for alternatelyturning on and oif successive ones of said plurality of digit switchesin timed relation providing a combined gain of the summing resistorswhich varies in discrete steps resembling a periodic staircase function.

4. The device of claim 3 in which said control means is furtherresponsive to the switching state of the last one of said successiveplurality of digit switches for turning off all of said digit switchesupon the turning-off of said last digit switch.

5. The device of claim 3, in which said control means includes periodicmeans for alternately turning on and off successive ones of theplurality of digit switches upon the concurrence of the off-states ofcombinations of preceding ones of the plurality of digit switches.

6. The device of claim 5 in which the gain of each of the attenuationnetworks associated with said successive switch means is substantiallyequal to the combined gains of the preceding ones plus the firstnetwork.

7. The device of claim 5 in which said successive switches arealternately turned olf and on at one-half the frequency of the precedingswitch.

8. The device of claim 7 in which said control means includes anindividual multivibrator controlling the setting of each of saidswitches and in which the gain weighting of each successive network isindicative of the period of the periodic output of an associatedmultivibrator.

9. The device of claim 5, in which said digit switch means includes acompensating switch and associated compensating networks operativelyconnected to said output terminal and responsive to said control meansfor providing like output impedances during both switching states ofsaid switch.

10. The device of claim 9 in which each said digit switch includescompensating means for providing a compensation output impedance in theoit state of such digit switch which is equal to the switching meansoutput impedance during the on state of such digit switch and whichfurther provides an output impedance in the on state of such digitswitch which is equal to the switching means output impedance during theott state of such digit switch.

11. In a monopulse receiver system having a periodic system trigger andwhich generates a video signal, the amplitude and time-phase of whichare indicative of target angle and target range respectively, anelectronic signal device having an output terminal for adjusting thegain of said video signal as a function of said time phase, comprising:a plurality of signal-attenuating networks, responsive to said videosignal, each network providing a mutually exclusive degree ofattenuation; a like number of summing resistors as attenuating networks,each summing resistor interconnecting an output of a mutually exclusiveone of said attenuating networks to said output terminal; a like numberof switches as attenuation networks, each switch connected across theoutput of a mutually exclusive one of said networks, a monostablecontrol multivibrator adapted to be connected to said periodic systemtrigger, and having a response state in response to said system triggerwhich has a duration of more than half the period of said periodictrigger; a freerunning multivibrator, a like number of seriallyinterconnected bi-stable multivibrators as switches less one, eachbi-stable multivibrator having a rst-state input and a second stateinput and at least a first state output, a rst one of said bi-stablemultivibrators having its lirst and second state inputs responsivelyconnected to a common output of said free-running multivibrator, the rstand second state input of each successive one of said bi-stablemultivibrator being responsively connected to a common output of thepreceding one of said bi-stable multivibrators, control meansresponsively connected to said control multivibrator for starting saidfree-running multivibrator, said control means being furtherresponsively connected to a last one of said successive bi-stablemultivibrators for stopping said free-running multivibrator in a pre-setstate, each of said bi-stable multivibrators and said free-runningmultivibrator operatively connected to drive a mutually exclusive one ofsaid switches, whereby the video gain provided by said combination ofnetworks is adjusted in discrete steps resembling a periodic staircasefunction.

12. The device of claim 11 in which the relative gain weighting of theswitched network associated with each multivibrator corresponds to theperiod of the periodic output from such multivibrator.

References Cited by the Examiner UNITED STATES PATENTS 2,474,875 7/ 1949White 343-5 2,498,381 2/1950 Smith 343-5 2,880,935 4/1959 Johnson328-160 XR 3,012,199 12/1961 Dorczak et al. 328-186 XR ROBERT H. ROSE,Primary Examiner.

DAVlD G. REDINBAUGH, Examiner.

.W l l

1. IN A MONOPULSE RECEIVER SYSTEM WHICH GENERATES A PERIODIC VIDEOSIGNAL HAVING A TIME-PHASE RELATIVE TO A PERIODIC SYSTEM TRIGGER SIGNAL,MEANS FOR ADJUSTING THE GAIN OF SAID VIDEO SIGNAL AS A FUNCTION OF SAIDTIME PHASE, COMPRISING: AN INPUT TERMINAL SUBJECTED TO SAID VIDEO SIGNALFROM THE SOURCE THEREOF, AN OUTPUT TERMINAL, A PLURALITY OF ATTENUATINGNETWORKS COMMONLY INTERCONNECTING SAID INPUT TERMINAL AND SAID OUTPUTTERMINAL, THE OUTPUT GAIN WEIGHTING OF EACH SUCCESSIVE ATTENUATINGNETWORK BEING LARGER THAN PRECEDING ONES OF SAID ATTENUATING NETWORKS, ASUMMING RESISTOR INTERPOSED BETWEEN THE OUTPUT OF EACH OF SAID NETWORKSAND SAID OUTPUT TERMINAL, A SWITCH INTERPOSED IN CIRCUIT CONTROLLING THEACTUATION OF EACH SAID ATTENUATING NETWORK, CLOCK MEANS RESPONSIVE TOSAID PERIODIC TRIGGER SIGNAL FOR SELECTIVELY TURNING ON AND OFF SAIDSWITCHES IN A PREDETERMINED MANNER DURING A PERIODIC TIME INTERVAL,WHEREBY THE VIDEO GAIN AT SAID OUTPUT TERMINAL IS VARIED IN DISCRETEINCREMENTS RESEMBLING A PERIODIC STAIRCASE FUNCTION.